An anti-reflection quantum cascade laser based on a tilted surface high-order grating

Abstract

The present invention discloses an anti-reflection quantum cascade laser based on a tilted surface high-order grating. The cross-section of the laser in the x-direction and the y-direction is a transverse structure, and is provided with an N-type electrode, an InP substrate, an n-type doped lower cladding, an n-type doped lower limiting layer, an active layer (the active layer includes multiple cascade periods, each cascade period includes an injection region and an active region), a p-type doped light limiting layer, a p-type doped cladding, a ridge waveguide layer, and a surface metal electrode layer in sequence from bottom to top; the longitudinal structure of the laser is the z-direction, which is composed of an FP resonant cavity formed by a natural cleavage surface of the laser and a tilted surface high-order grating located in the FP resonant cavity; the z-direction is the optical resonance direction of the laser. The present invention has the advantages of simple structure, simple grating manufacturing process, low cost (compared with ordinary gratings), etc., and can improve the working stability and reliability of the quantum cascade laser after coupling and packaging with a micro-optical element.

Description

Technical Field

[0001] The invention belongs to the technical field of lasers, and in particular relates to an anti-reflection quantum cascade laser based on a tilted surface high-order grating. Background Art

[0002] Quantum cascade laser (QCL) is a unipolar light source based on electron resonant tunneling and inter-subband transitions in the conduction band in coupled multi-quantum wells or superlattices. It has the advantages of small size and easy wavelength adjustment. It is an innovative product that combines band engineering and molecular beam epitaxy preparation technology. Over the past 20 years, QCL technology has made rapid progress, especially in the mid-wave infrared band, and has basically reached the level of practical application. Quantum cascade lasers have important value and significance in the civilian and military fields. Military field: Quantum cascade lasers can be used as infrared bait to confuse and change the path of guided missiles; they can be used as a light source for long-range active laser detection by the army. In civilian fields such as trace gas detection, gas detection devices using quantum cascade lasers as light sources have the advantages of high detection sensitivity, fast response speed, and flexible gas selectivity. Quantum cascade lasers will surely occupy an important position in the future laser industry.

[0003] At present, quantum cascade lasers generally adopt an edge-emitting structure, that is, the light-emitting surface is the end face of the waveguide, parallel to the direction of the semiconductor layer. The cavity surface of the edge-emitting laser is generally the cleavage surface of the semiconductor crystal. Since the cleavage surface of the crystal is very flat and smooth, it is a good reflective surface, so the two parallel cleavage end faces of the laser naturally form the resonant cavity surface of the edge-emitting laser (that is, the light-emitting cavity surface and the reflection cavity surface). Quantum cascade lasers need to be packaged for practical application. During the packaging process, quantum cascade lasers need to be coupled with optical microlenses to improve their beam characteristics. The back-reflected light generated by the quantum cascade laser when coupled with the microlens will be reflected into the resonant cavity of the quantum cascade laser, interfering with the normal operation of the laser and having a serious impact on the performance and reliability of the laser. For high-power quantum cascade lasers, this reflected light has a more serious impact: because high-power quantum cascade lasers usually work in multi-mode, the lasing mode itself is extremely unstable during use, and mode hopping is prone to occur. External reflected light will greatly aggravate this phenomenon, which is easy to cause damage to the laser. If a new structure design can be made in the quantum cascade laser itself to reduce the interference of external reflected light, it will be beneficial to improve the working stability and service life of the quantum cascade laser, increase the compactness of the quantum cascade laser package (minus the anti-reflection element), and ultimately reduce the overall packaging cost. Summary of the invention

[0004] In order to solve the above technical defects, the purpose of the present invention is to provide an anti-reflection quantum cascade laser based on a tilted surface high-order grating, which solves the problems of unstable working mode, reduced service life and easy damage of quantum cascade lasers due to the influence of external reflected light. Compared with traditional edge-emitting quantum cascade lasers, this laser has better mode performance, working stability, smaller packaging volume and lower comprehensive packaging cost.

[0005] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: an anti-reflection quantum cascade laser based on a tilted surface high-order grating, the cross section of the laser in the x-direction and the y-direction is a lateral structure, and is provided with an N-type electrode, an InP substrate, an n-type doped lower cladding layer, an n-type doped lower limiting layer, an active layer (the active layer includes a plurality of cascade periods, each cascade period includes an injection region and an active region), a p-type doped light limiting layer, a p-type doped cladding layer, a ridge waveguide layer and a surface metal electrode layer in order from bottom to top; The longitudinal structure of the laser, i.e. the z direction, is composed of an FP resonant cavity formed by the natural cleavage surface of the laser and an inclined surface high-order grating located in the FP resonant cavity; the z direction is the optical resonance direction of the laser, and the FP resonant cavity is a cavity in which the broadcast is reflected back and forth to provide light energy, and is composed of a laser working medium and two end surface reflection surfaces; the inclined surface high-order grating has a certain angle θ with the y direction.

[0006] Furthermore, the inclined surface high-order grating of the anti-reflection quantum cascade laser is rectangular in shape and has a length of 100 μm to 350 μm; the grating period of the inclined surface high-order grating is variable and is set as a "chirped" grating with a variable grating period.

[0007] The order of the inclined surface high-order grating is high-order, that is, the grating period of the N-order grating is N times the period of the ordinary Bragg grating, and the grating order is set to 3rd order, 5th order, or 7th order; the grating period of the inclined surface high-order grating is 1200nm~2800nm, and this grating period range can be produced using ordinary photolithography technology with low production difficulty; the duty cycle of the inclined surface high-order grating is 20%~90%, and the depth is 0.1μm~3μm.

[0008] The inclined surface high-order grating of the anti-reflection quantum cascade laser is a surface grating, whose lateral structure covers an area extending from the ridge waveguide layer of the laser to above the active layer, and whose longitudinal structure is located in the middle area of ​​the laser resonant cavity.

[0009] The inclined surface high-order grating has a certain angle θ with the y direction, that is, the grating is not perpendicular to the propagation direction of light in the anti-reflection quantum cascade laser, and the range of θ is 3° to 10°.

[0010] The active layer of the anti-reflection quantum cascade laser includes multiple cascade periods, each cascade period includes an electron injection region and an active region; the number of cascade periods of the active region and the electron injection region is greater than 15; the material composition of the active layer is strained InGaAs / InAlAs quantum wells with different material ratios and doping concentrations.

[0011] The resonant cavity length of the anti-reflection quantum cascade laser is 2 to 5 mm; the width of the laser ridge waveguide is 5 to 20 μm.

[0012] The resonant cavity length of the anti-reflection quantum cascade laser is 3 mm; the width of the laser ridge waveguide is 10 μm.

[0013] Compared with the existing design solutions, the design scheme conceived by the present invention has the following advantages: The manufacturing process steps of the designed anti-reflection quantum cascade laser are basically the same as those of the mature ordinary ridge waveguide strip FP cavity quantum cascade laser. It only needs to add the production of inclined surface high-order grating on its basis; the grating period of the inclined surface high-order grating is N times (N is the grating order) of the first-order grating (Bragg grating) period of the traditional quantum cascade laser. The advantage of the high-order grating is that it has a long period and can be manufactured using low-cost ordinary photolithography technology. The manufacturing process is simple, the process tolerance is large, and the cost is low. In addition, the production of inclined surface gratings does not require precise grating writing technology, and can be produced on a large scale using nanoimprinting technology, which is suitable for large-scale commercial applications.

[0014] Compared with the quantum cascade laser using optical isolator, the designed quantum cascade laser has the advantages of simple structure and flexible design. The surface grating has a mode selection effect, which improves the overall mode performance of the laser.

[0015] Compared with the anti-reflection laser realized by using buried partial feedback grating structure, the designed quantum cascade laser does not require secondary epitaxy on the surface grating structure, and the process complexity is reduced; the high-order surface grating has a long period, can be deeply etched, and has strong resistance to external damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic diagram of the epitaxial structure of a surface high-order grating anti-reflection quantum cascade laser provided in an embodiment of the present invention.

[0017] Figure 2 A schematic longitudinal cross-sectional view of a surface high-order grating anti-reflection quantum cascade laser provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0019] See attached Figure 1-2 An embodiment of the present invention discloses an anti-reflection quantum cascade laser based on a tilted surface high-order grating. The cross-section of the laser in the x-direction and the y-direction is a lateral structure, and is provided with an N-type electrode 1, an InP substrate 2, an n-type doped lower cladding 3, an n-type doped lower limiting layer 4, an active layer 5, a p-type doped light limiting layer 6, a p-type doped cladding 7, a ridge waveguide layer 8 and a surface metal electrode layer 9 in sequence from bottom to top.

[0020] The longitudinal structure of the laser, i.e., the z direction, is composed of an FP resonant cavity formed by the natural cleavage surface of the laser and an inclined surface high-order grating located in the FP resonant cavity; the z direction is the optical resonance direction of the laser, and the FP resonant cavity is a cavity in which the broadcast is reflected back and forth to provide light energy, and is composed of a laser working medium and two end surface reflection surfaces; the inclined surface high-order grating has a certain angle θ with the y direction, that is, the grating is not perpendicular to the propagation direction of the light in the anti-reflection quantum cascade laser, and the θ value is set to 5°.

[0021] The tilted surface high-order grating of the anti-reflection quantum cascade laser is rectangular in shape and 250 μm in length; the grating period of the tilted surface high-order grating is variable and is set as a "chirped" grating with a variable grating period.

[0022] The order of the inclined surface high-order grating is high-order, that is, the grating period of the N-order grating is N times the period of the ordinary Bragg grating, and the grating order is set to 5 orders; the grating period of the inclined surface high-order grating is 2000nm, and this grating period range can be produced using ordinary photolithography technology with low production difficulty; the duty cycle of the inclined surface high-order grating is 50%, and the depth of the grating is 1μm.

[0023] The inclined surface high-order grating of the anti-reflection quantum cascade laser is a surface grating, whose lateral structure covers an area extending from the ridge waveguide layer of the laser to above the active layer, and whose longitudinal structure is located in the middle area of ​​the laser resonant cavity.

[0024] The active layer of the anti-reflection quantum cascade laser includes multiple cascade periods, each cascade period includes an electron injection region and an active region; the number of cascade periods of the active region and the electron injection region is set to 16; the material composition of the active layer is strained InGaAs / InAlAs quantum wells with different material ratios and doping concentrations.

[0025] The resonant cavity length of the anti-reflection quantum cascade laser is 3 mm; the width of the laser ridge waveguide is 10 μm.

[0026] The substrate is the base of the epitaxially grown laser. The active layer contains multiple cascade cycles, each of which is composed of an injection region and an active region. Under the action of an external electric field, after electrons are injected into the active region of one cycle and stimulate photons, they will continue to be injected into the active region of the next cycle and stimulate it to emit photons. The optical confinement layer located above and below the active layer is used to confine the laser in the vertical direction to prevent the laser in the active layer from leaking out of the laser. The optical confinement layer greatly improves the light extraction efficiency of the laser. Unlike traditional lasers, our laser is designed with an inclined high-order surface grating, which is located above the active layer and extends from the ridge waveguide layer to the p-type optical confinement layer, preferably a third-order surface grating.

[0027] Figure 2 The figure shows a longitudinal cross-sectional diagram of a tilted surface high-order grating anti-reflection quantum cascade laser along the resonant cavity direction. Figure 1 A screenshot of the left view. In the figure, the z direction is the longitudinal direction of the laser, and the x direction is the epitaxial (vertical) direction of the laser. A grating layer is separated from the active layer.

[0028] The surface metal electrode layer 9 is used for injecting external current (the laser needs current to work).

[0029] In the embodiment of the present invention, the material selected for the laser is a III-V group material, the central wavelength of the laser emission is selected to be 4.6 μm, the thickness of each layer of the laser is a typical value, and the length of the laser is preferably 900 μm.

[0030] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An anti-reflection quantum cascade laser based on a tilted surface high-order grating, characterized in that: The cross section of the laser in the x-direction and the y-direction is a lateral structure, and is provided with an N-type electrode, an InP substrate, an n-type doped lower cladding layer, an n-type doped lower limiting layer, an active layer (the active layer includes a plurality of cascade periods, each cascade period includes an injection region and an active region), a p-type doped optical limiting layer, a p-type doped cladding layer, a ridge waveguide layer and a surface metal electrode layer in order from bottom to top; The longitudinal structure of the laser, i.e. the z direction, is composed of an FP resonant cavity formed by the natural cleavage surface of the laser and an inclined surface high-order grating located in the FP resonant cavity; the z direction is the optical resonance direction of the laser, and the FP resonant cavity is a cavity in which the broadcast is reflected back and forth to provide light energy, and is composed of a laser working medium and two end surface reflection surfaces; the inclined surface high-order grating has a certain angle θ with the y direction.

2. The anti-reflection quantum cascade laser based on inclined surface high-order grating according to claim 1, characterized in that: The tilted surface high-order grating of the anti-reflection quantum cascade laser is rectangular in shape and has a length of 100 μm to 350 μm; the grating period of the tilted surface high-order grating is variable and is set as a "chirped" grating with a variable grating period.

3. The anti-reflection quantum cascade laser based on inclined surface high-order grating according to claim 1 or 2, characterized in that: The order of the inclined surface high-order grating is high-order, that is, the grating period of the N-order grating is N times the period of the ordinary Bragg grating, and the grating order is set to 3rd order, 5th order, or 7th order; the grating period of the inclined surface high-order grating is 1200nm~2800nm, and this grating period range can be produced using ordinary photolithography technology with low production difficulty; the duty cycle of the inclined surface high-order grating is 20%~90%, and the depth is 0.1μm~3μm.

4. The anti-reflection quantum cascade laser based on a tilted surface high-order grating according to any one of claims 1 to 3, characterized in that: The inclined surface high-order grating of the anti-reflection quantum cascade laser is a surface grating, whose lateral structure covers an area extending from the ridge waveguide layer of the laser to above the active layer, and whose longitudinal structure is located in the middle area of ​​the laser resonant cavity.

5. The anti-reflection quantum cascade laser based on inclined surface high-order grating according to claim 1, characterized in that: The inclined surface high-order grating has a certain angle θ with the y direction, that is, the grating is not perpendicular to the propagation direction of light in the anti-reflection quantum cascade laser, and the range of θ is 3° to 10°.

6. The anti-reflection quantum cascade laser based on inclined surface high-order grating according to claim 1, characterized in that: The active layer of the anti-reflection quantum cascade laser includes multiple cascade periods, each cascade period includes an electron injection region and an active region; the number of cascade periods of the active region and the electron injection region is greater than 15; the material composition of the active layer is strained InGaAs / InAlAs quantum wells with different material ratios and doping concentrations.

7. The anti-reflection quantum cascade laser based on inclined surface high-order grating according to any one of claim 1, characterized in that: The resonant cavity length of the anti-reflection quantum cascade laser is 2 to 5 mm; the width of the laser ridge waveguide is 5 to 20 μm.

8. The anti-reflection quantum cascade laser based on inclined surface high-order grating according to claim 7, characterized in that: The resonant cavity length of the anti-reflection quantum cascade laser is 3 mm; the laser ridge waveguide width is 10 μm.

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